Various kinds of construction for a suspension for a vehicle such as an automobile are known. Recently, a double wishbone suspension, which has a high degree of design freedom and has excellent road surface followability, is used in many kinds of automobiles such as luxury automobiles and sports cars. FIG. 4 illustrates a first example of conventional construction of a double wishbone suspension that is disclosed in “Illustrated Book of Car Mechanisms and Structures” (Hosokawa, Takeshi; Grand Prix Book Publishing Co., Ltd.; Jan. 10, 2003; pg. 207).
A knuckle 3, which supports a wheel 1 by way of a bearing unit 2 so as to be able to rotate, is supported by the vehicle (not illustrated in the figure) by an upper arm 4 and a lower arm 5 of a double wishbone suspension so as to be able to pivot. The upper arm 4 is made using a so-called A-frame having an A shape, with the tip end section (when installed in the vehicle, the end section on the outside in the width direction of the vehicle, or the end section on the right side in FIG. 4) thereof being connected to the top end section of the knuckle 3 by way of an upper ball joint 6. Moreover, the base end section of the upper arm 4 (when installed in the vehicle, the end section on the center side in the width direction of the vehicle, or the end section on the left side in FIG. 4) is supported by the vehicle (not illustrated in the figure) by a pivot shaft so as to be able to pivot.
On the other hand, the lower arm 5 is also made using so-called A-frame having an A shape, with the tip end section thereof being connected to the bottom end section of the knuckle 3 by way of a lower ball joint 7. Moreover, the base end section of the lower arm 5 is supported by the vehicle (not illustrated in the figure) by a pivot shaft so as to be able to pivot. Furthermore, the lower arm 5 supports the bottom end section of a shock absorber 8, the top end of which is fastened to the vehicle, by a pivot shaft so as to be able to pivot.
In the case of the double wishbone suspension having this kind of conventional construction, by using an upper arm 4 and lower arm 5 having different overall lengths (typically the lower arm is longer than the upper arm), the camber angle is preset to a specified angle. However, in the case of a suspension having conventional construction, the camber angle that is preset in this way cannot be changed according to the traveling conditions of the vehicle.
Incidentally, the turning movement of a vehicle occurs due to the difference in drive force on the left and right wheels and the like, however mainly occurs due to lateral force on the tires. This lateral force on the tires is generated by the driver operating the steering wheel and causing the toe angle (steering angle) of the front wheels to change by way of a steering gear, and causing a shift (slip angle) to occur between the traveling direction of the vehicle and the direction of the tires. This lateral force on the tires, in addition to the toe angle, is known to be affected by the change in the camber angle. FIG. 5 illustrates the relationship between the lateral force on the tires and the slip angle when the camber angle, which was found by the inventors through simulation, was used as a parameter. As can be clearly seen in FIG. 5, even when the slip angle is constant, it is possible to cause the lateral force on the tires to change by causing the camber angle to change. Therefore, by being able to adjust the size of the lateral force on the tires by changing the camber angle, it is possible to improve the turning performance of the vehicle, as well as further improve the performance when going straight.
JP 10-264636 (A) discloses a double wishbone suspension for a vehicle that is capable of changing the camber angle according to the traveling condition of the vehicle. FIG. 6 illustrates a second example of conventional construction as disclosed in this patent literature. In the case of this second example of conventional construction, hydraulic cylinders 9 and 10, which are capable of expanding and contracting, are provided in the middle section of the upper arm 4a and lower arm 5a and make it possible for the entire length of upper arm 4a and the lower arm 5a to change. A sensor (not illustrated in the figure) detects the angle of sideslip of the wheel 1, and when it is recognized that the camber angle must be changed, a specified amount of hydraulic oil is supplied to the hydraulic cylinders 9 and 10 from a hydraulic pump that is located in the engine room by way of hydraulic piping and various valves. As a result, the entire lengths of the upper arm 4a and the lower arm 5a are changed and the camber angle of the wheel 1 is changed. With the suspension of this second example of convention construction, it is possible to adjust the size of the lateral force on the tires, and improve the turning performance of the vehicle, as well as the performance for traveling straight.
However, in the case of this second example of conventional construction, in order to make it possible to change the camber angle, it is necessary to provide a hydraulic pump, hydraulic piping and various valves, and it is also necessary to provide hydraulic cylinders 9, 10 in the upper arm 4a and lower arm 5a. Therefore, not only is there a problem in that the construction for making it possible to change the camber angle is complex, but there is also a problem in that the size and weight of the suspension are increased. Particularly, the increase in the weight of the upper arm 4a and the lower arm 5a is connected to an increase in the unsprung weight, which is undesirable from the aspect of improve traveling performance of the vehicle centered on comfort and traveling stability. Moreover, control of the camber angle is performed hydraulically, so there is a problem in that the controllability and response is bad, and furthermore there is a problem in that power loss of the engine becomes large.
JP 2009-107533 (A) and JP 2010-83212 (A) disclose camber angle adjustment apparatuses for changing the size of the camber angle according to the traveling conditions of the vehicle. However, even in the case of the apparatuses disclosed in these patent literatures, as in the case of the apparatus disclosed in JP 10-263636 (A), there is a problem in that the construction for making it possible to change the camber angle is complex, and invites an increase in size and weight of the suspension. As related technology to the present invention, there is a load measurement apparatus for measure a load applied to a bearing unit that is disclosed in JP 2005-98771 (A).